Improvement of the service life of a cutting tool has been carried out by vapor deposition of a coating layer of titanium carbide (TiC) on a surface of cemented carbides, cermets or ceramics and in general, a coated cutting tool having a coating layer formed by a thermal or heat-assisted chemical vapor deposition method (hereinafter referred to as heat-assisted CVD method) or plasma CVD method has widely been spread.
However, in the case of carrying out a working using these coated cutting tools, for example, working which requires wear resistance of the coating layer at high temperatures, such as high speed cutting, and a working requiring a number of workings and a number of thrusting-in workpieces, such as the working of small-sized parts, results in a decrease in service life of the tool resulting from the poor wear resistance of the coating layer or damaging of the coating layer.
In the coating film by the heat-assisted CVD method, the adhesiveness thereof to a substrate is excellent, and depending upon the variety of the substrates, the .eta. phase as a brittle layer tends to be deposited thick on the interface with the substrate, in particular, near the cutting edge ridge line. During cutting, the coating layer falls away with this .eta. phase to promote wearing of the tool and a reduction in the service life of the tool. Thus, the presence of the coating layer does not necessarily result in the improvement in the wear of the tool.
In these coated cutting tools, factors affecting the wear resistance and peeling resistance are the chlorine content in components for forming the coating layer, and the preferred orientation.
Coating of titanium carbide or titanium nitride (TiN) by the heat-assisted CVD method is generally carried out by the use of titanium tetrachloride (TiCL.sub.4) as a titanium source, methane (CH.sub.4) as a carbon source and nitrogen gas as a nitrogen source. Thus, in the coating using these gases, chlorine resulting from the titanium tetrachloride is taken in the coating layer, resulting in deterioration of the film quality.
As a report as to the chlorine in the film, there are disclosed techniques in which coating is carried out at a low temperature side using a plasma CVD method, for example, in "Hyomen Gijitsu (Surface Techniques)" Vol. 40, No. 10, 1889, p 51-55, "Hyomen Gijitsu" Vol. 40, No. 4, 1889, p 33-36, etc. This report tells that the level of the chlorine content in the film can be reduced to about 1 atom % by film making by the plasma CVD method at a temperature of up to 700.degree. C., whereby good film quality can be obtained.
Japanese Patent Laid-Open Publication. No. 13874/1992 describes that a titanium carbide film with excellent film adhesiveness as well as good wear resistance is composed of two titanium coating layers consisting of one having a chlorine content of 0.025 to 0.055 atom % up to a thickness of less than 0.5 .mu.m from the surface of a substrate and the other having a chlorine content of 0.055 to 1.1 atom % at a thickness of at least 0.5 .mu.m therefrom. In the method described in this publication, since titanium tetrachloride is used as a raw material gas and free carbon (C) from methane is used as a carbon source, the chlorine (Cl) from the titanium tetrachloride and the free carbon from the methane are taken in the film, thus unfavourably affecting the property of the film. In particular, precipitation of C in the film lowers the wear resistance, which should preferably be avoided, but the presence of chlorine in a proportion of at least 0.055 atom % suppresses the precipitation of C to give a titanium carbide film excellent in wear resistance without depositing carbon. According to this method, therefore, it is required that the chlorine content is adjusted to 0.025 to 0.055 atom % near the substrate interface and to at least 0.055 atom % far from the interface so as to increase the adhesiveness. In this case, moreover, the presence of chlorine itself causes lowering of the wear resistance, so the wear resistance of the resulting film cannot be said to be sufficient.
When an intermittent working or parts working is carried out using a coated cutting tool provided with a coating layer by a heat-assisted CVD method, up to the present time, there have occurred separation of a substrate and a film and breakage of a film itself, whereby exposure or breakage of the substrate has often been caused, and as one of the causes of breakage of the film itself, the preferred orientation of the coating layer is considered. Ordinarily, it is known that a coating layer of titanium carbide, etc. by the heat-assisted CVD method is strongly orientated to (220) plane "Nippon Kinzoku Gakkai-shi (Journal of Japanese Metallurgical Society)" Vol. 41, No. 6, 1977, p 542-545!, but in the titanium carbide, etc. having the rock salt type structure, (220) plane is a primary slip plane at a temperature of at most about 600.degree. C. corresponding to the edge temperature of a cutting edge in such a working, and breaking tends to occur towards the direction of this plane. In addition, in the vicinity of the interface with the substrate, such a large tensile residual stress due to difference in thermal expansion coefficient between the substrate and coating layer is applied to the coating layer that when the cutting tool is rubbed in a parallel direction to the film surface by a workpiece or chips during working and a shearing stress is thus applied to the film, the film is considered to be breakable in the vicinity of the interface with the substrate.
As a means for solving the above described problem on the .eta. phase, a process for forming a titanium carbonitride film by a heat-assisted CVD method using an organo CN compound such as acetonitrile (CH.sub.3 CN), etc. has been watched with keen interest (Japanese Patent Laid-Open Publication No. 117809/1975, Japanese Patent Laid-Open Publication No. 109828/1975, etc.)
According to this process, coating is rendered possible at a lower temperature as compared with the heat-assisted CVD method, so this is generally called "medium temperature CVD method (MT-CVD method)". In the prior art heat-assisted CVD method (high temperature CVD method; called "HT-CVD method"), elements, in particular, carbon is transferred to the film from the substrate during forming a titanium-containing film to form a modified layer (composite carbides such as Co.sub.3 W.sub.3 C, etc. called .eta. phase). The transfer of elements in the HT-CVD method as described above is probably due to that the coating temperature is higher (ordinarily 1000 to 1050.degree. C.). As to the transfer of carbon, in particular, it is considered, in addition to the high temperature, that such a phenomenon takes place, for example, that a concentration gradient of carbon is formed between the substrate surface and film during being formed because of insufficient supply of carbon from the gaseous phase during forming the film and the film absorbs carbon from the substrate.
In the MT-CVD method, on the other hand, it is considered that no .eta. phase is formed even at the interface of the cutting edge ridge line part, because the coating temperature is somewhat lower (800 to 900.degree. C.) and supply of C and N from the gaseous phase is sufficient.
Since then, a number of patent applications have been filed employing the MT-CVD method. For example, Japanese Patent Laid-Open Publication Nos. 64469/1991 and 87368/1991 have respectively proposed a tool obtained by directly forming a titanium carbonitride (TiCN) film on a surface of a cemented carbide substrate by the MT-CVD method and forming a multi-layer film of alumina (Al.sub.2 O.sub.3) or titanium nitride (TiN) by the HT-CVD method. In Japanese Patent Laid-Open Publication Nos. 99467/1987, there are disclosed a single layer or laminated layer film consisting of a titanium carbonitride film and/or titanium nitride film each having a crystalline grain diameter of at most 0.5 .mu.m, coated with a thickness of 0.5 to 5.0 .mu.m, and a process comprising forming a titanium carbonitride film by the MT-CVD method at a vapor deposition temperature of 700 to 900.degree. C. In this process, however, the film in contact with a substrate is a titanium carbonitride film (TiCN).
While the inventors have been making studies on a cemented carbide substrate coated with a titanium carbonitride film, it is found that the adhesiveness of the titanium carbonitride film by the MT-CVD method to the cemented carbide substrate often becomes unstable. Thus, the inventors have made analysis of this phenomenon and consequently, have found that this is caused by etching of cobalt (Co) as a binder phase on the surface of the cemented carbide substrate with chlorine gas formed as a reaction product during forming the titanium carbonitride film by the MT-CVD method.
The thermal decomposition of an organo CN compound such as acetonitrile is susceptible to the chemical bonded state of the substrate surface and often lead to formation of free carbon. The thus formed free carbon lowers the adhesiveness of the film and substrate and renders unstable the property of the coated cutting tool by the MT-CVD method in combination with the foregoing formation of the interface modified layer.
Japanese Patent Laid-Open Publication Nos. 170559/1986 discloses a surface-coated cemented carbide comprising a cemented carbide substrate the surface of which is coated with a multi-layer of titanium carbide, titanium nitride and titanium carbonitride, the innermost layer in contact with the substrate consisting of titanium nitride with a thickness of 0.1 to 1.0 .mu.m. This is related with a coating by a PVD method and is silent as to the effects of the chlorine content in the film and the preferred orientation of the crystal.
As to the hardness of the film, it is generally considered that the higher the hardness, the more excellent the wear resistance, but there arises a problem that when only the hardness of the film is high, the toughness of the film is lowered to be brittle and in the case of a cutting tool, it tends to meet with abnormal wear and cannot practically be used. Accordingly, it has been required to balance the hardness and toughness properties.
As to the micro structure of the film, it has been proposed, as described in Japanese Patent Laid-Open Publication Nos. 99467/1987, that a titanium carbonitride and/or titanium nitride film having a crystal grain diameter of at most 0.5 .mu.m is most suitable for making up a coating layer, but this proposal is not practical because of no description of an assessment method regarding the shape of the crystal grain and grain diameter.
It is an object of the present invention to provide a coated cutting tool with a higher wear resistance, more tenacious adhesiveness of a coating film and substrate and more excellent peeling or stripping resistance during cutting as compared with coated cutting tools of the prior art, and a process for the production of the same, whereby the foregoing problems in the prior art can be solved.
Furthermore, it is another object of the present invention to provide a coated cutting tool with higher reliability than cutting tools of the prior art, whereby the advantages of the titanium carbonitride film by the MT-CVD method can be given to the maximum extent.
In order to achieve these objects, the present invention provides a coated cutting tool having a film structure capable of preventing the surface of a substrate from modification during forming the coating and inhibiting deposition of an unfavourable material on the interface between the film and substrate, and provides a coated cutting tool having not only a macro film structure optimized but also a structure optimized from the standpoint of the microscopic structure and the hardness, and a mechanical strength in an optimum range.